1. Field of the Invention
The present invention relates generally to an improved small-sized disk unit, and more particularly to a connection structure by means of FPC's (flexible printed circuits) between a head and an IC, a RAM storage of various types of offset correction values, position sensitivity measurement processing and a small-sized disk unit which generates sector pulses allowing for encode loss in write operation and decode loss in read operation.
2. Description of the Related Art
With a recent remarkable reduction in cost of magnetic disk units, cost-effective production and assembly are also pressingly desired. In conventional techniques, connection is made as follows between a head IC mounted FPC which is disposed on a base and a head which is supported at the distal end of a head actuator. A flexible read/write FPC band is extended from the head IC mounted FPC. A relay FPC extending from the head on the other hand is adhesively joined to the lateral surface of a head arm of the actuator. Land areas with terminal patterns of the two FPC's are fixedly joined together in a superposed manner on the lateral surface of the head arm. In the conventional FPC attachment structure, however, since the terminal ends of two FPC's are electrically and mechanically connected together in a superposed manner on a flat mounting surface of the head arm, the land area of the FPC on one hand is adhered securely to the mounting surface whereas the land area of the FPC on the other is superposed thereon in a raised state relative to the mounting surface. In this manner, connection surfaces of the two FPC's do not lie in the same plane, which may subject the land areas to a positional offset upon the superposition, thus necessitating a high-precision positioning and increasing the number of working steps. This may preclude a sufficient curtailment in production and assembly costs.
It is also anticipated that the magnetic disk unit may be used in a place subjected to an extreme variation in environmental temperature and humidity, and hence various types of head position corrections are effected in order to ensure a normal operation. In this case, closer positions of cylinders subjected to such head corrections will lead to a higher precision correction. In order to provide a closer cylinder positions to secure a higher precision correction, however, the capacity of the RAM for storing correction values therein must be larger, which will prevent the costs from being lowered. It is thus desired to provide a correction capable of reducing the use capacity of the RAM without impairing the precision of correction. In the conventional magnetic disk unit, the entire cylinder range of a disk medium is divided into a plurality of zones at equal intervals, and-correction values at zone boundaries are measured and stored in the memory. Correction values for arbitrary positions within a zone whose correction values have not been measured and stored are found by means of a linear interpolation from the correction values at opposite zone boundaries which have been stored. However, the correction values for the cylinder positions may often present non-linear characteristics. In the case of dividing into a plurality of zones at equal intervals, the zone intervals must be closer in order to ensure an accurate measurement of the non-linear portion. This will result in closer zone intervals in the linear portion and hence an increase in capacity of the memory for storing the measured correction values, thus bringing about an increase in costs. For the realization of cost-cut, in recent years in particular, a reduced capacity RAM is used as an internal memory of an MCU for use in a disk controller. This has a smaller RAM area available for the storage of the correction values. Efficient storage of the correction values is thus desired.
On the contrary, recent magnetic disk units tend to employ a closer track pitch with the reduction in size and increase in capacity, and hence the improvement in on-track precision is desired. Also, due to the employment of an MR head having a small core width as the read head in contrast with the write head using an inductive head, a core offset correction is inevitable between the write head and the read head. Thus, a phase variation position may become an on-track position. For this reason, in place of the conventional phase variation of two-phase servo pattern at a half track pitch, two-phase servo information is proposed in which the phase varies at one-third pitch. In order to effect the measurement of position sensitivity correction values in the conventional disk unit, it is necessary to measure a cross point of the two-phase servo signals N and Q. It is however impossible to directly measure the cross point since the servo signals N and Q are discretely obtained for each of sample cycles. A value of the cross point has thus been determined by means of a linear interpolation from values anterior and posterior to crossing the cross point of the two-phase servo signals N and Q which can be obtained when performing an equal-speed seek. An error may therefore be involved therein. In the case of the two-phase servo signals in which the track pitch is reduced to allow the phase to vary at one-third track pitch, two cross points appear during one track displacement. This means that the cross point density is doubled as compared with the case of a half track pitch. Thus, too much time is disadvantageously required for the measurement of the position sensitivity correction values, and it was difficult to expect to improve the measurement precision due to the linear interpolation.
To implement a size reduction and capacity increase of the disk unit, miniaturization of the disk size is advanced with the employment of disk high density recording for the increase in capacity. Also, for the read and write signal processing system, a partial response most likelihood (PRML) method is employed to heighten the function. With such heightening of function of the signal processing system, encode and decode time loss which could have been neglected for the conventional 1-7RLL, etc. tends to be increased. In the 1-7RLL for example, it was merely of the order of five bits. However, the partial response most likelihood method based signal processing entails as much as 44-bit loss, which is about ten times the former loss. A gap region for accommodating the loss must be provided and hence the format efficiency may be lowered. In the case of a conventional format allowing for both the encode loss and decode loss, in particular, the format efficiency will be remarkably lowered since it is provided with a gap region corresponding to the sum of the encode loss and the decode loss, the encode loss meaning an elongation of time taken in write operation after the completion of NRZ data input until the completion of write into the disk medium, the decode loss meaning an elongation of time taken in read operation after the acquisition of read signal until the actual NRZ data demodulation output.